Re: [PATCH v3 1/3] net: dsa: microchip: implement KSZ87xx Module 3 low-loss cable errata

[Andrew Lunn <andrew@lunn.ch>]

On Tue, Apr 14, 2026 at 03:48:33PM +0200, Fidelio LAWSON wrote:
> On 4/14/26 14:40, Andrew Lunn wrote:
> > On Tue, Apr 14, 2026 at 01:05:49PM +0200, Marek Vasut wrote:
> > > On 4/14/26 11:12 AM, Fidelio Lawson wrote:
> > > > Implement the "Module 3: Equalizer fix for short cables" erratum from
> > > > Microchip document DS80000687C for KSZ87xx switches.
> > > > 
> > > > The issue affects short or low-loss cable links (e.g. CAT5e/CAT6),
> > > > where the PHY receiver equalizer may amplify high-amplitude signals
> > > > excessively, resulting in internal distortion and link establishment
> > > > failures.
> > > > 
> > > > KSZ87xx devices require a workaround for the Module 3 low-loss cable
> > > > condition, controlled through the switch TABLE_LINK_MD_V indirect
> > > > registers.
> > > > 
> > > > The affected registers are part of the switch address space and are not
> > > > directly accessible from the PHY driver. To keep the PHY-facing API
> > > > clean and avoid leaking switch-specific details, model this errata
> > > > control as vendor-specific Clause 22 PHY registers.
> > > > 
> > > > A vendor-specific Clause 22 PHY register is introduced as a mode
> > > > selector in PHY_REG_LOW_LOSS_CTRL, and ksz8_r_phy() / ksz8_w_phy()
> > > > translate accesses to these bits into the appropriate indirect
> > > > TABLE_LINK_MD_V accesses.
> > > > 
> > > > The control register defines the following modes:
> > > > 0: disabled (default behavior)
> > > > 1: EQ training workaround
> > > > 2: LPF 90 MHz
> > > > 3: LPF 62 MHz
> > > > 4: LPF 55 MHz
> > > > 5: LPF 44 MHz
> > > I may not fully understand this, but aren't the EQ and LPF settings
> > > orthogonal ?
> > 
> > What is the real life experience using this feature? Is it needed for
> > 1cm cables, but most > 1m cables are O.K with the defaults? Do we need
> > all these configuration options? How is a user supposed to discover
> > the different options? Can we simplify it down to a Boolean?
> We were seeing random link dropouts with the default settings, and since
> enabling the workaround 2, the link has remained stable and we have not
> observed any further issues.

So for you, a boolean which enables workaround 2 would be sufficient.

Marek, what is your experience?

       Andrew